Fiber-reinforced composite materials, in particular, carbon-reinforced composite materials are excellent in specific strength and specific rigidity to be useful, and the use thereof has been developing widely for airplane structural members, blades of a windmill, outside plates of an automobile, and members for computers, such as IC trays or laptop enclosures (housings). Demands therefor have been increasing year by year.
A fiber-reinforced composite material is a heterogeneous material obtained by molding a prepreg containing, as essential materials, a reinforcement fiber and a matrix resin. Thus, a large difference exists in physical properties between the array direction of the reinforcement fiber and any other direction. For example, it is known that the composite material is not drastically improved in interlaminar fracture toughness, which shows the difficulty degree of the advance of a destruction of reinforcement-fiber-layers of the material between these layers, only by improving the strength of the reinforcement fiber. In particular, a carbon fiber reinforced composite material containing a thermosetting resin as a matrix resin has a property of being easily destroyed by a strain along any direction other than the array direction of the reinforcement fiber, this matter being reflected on a low rigidity of the matrix resin. Thus, various techniques have been suggested to improve physical properties of a composite material that are capable of coping with a strain along any direction other than the array direction of the reinforcement fiber, a typical example of the physical-properties being interlaminar fracture toughness, while the composite material keeps a compression strength in a high-temperature and high-humidity environment, this strength being required, in particular, for airplane structural members. For example, many techniques are disclosed for improving a composite material in compression strength after impact, which is particularly required for airplane structural members.
Furthermore, in recent years, airplane structural moieties to each of which a fiber-reinforced composite material is applied have been increasing. Additionally, the application of a fiber-reinforced composite material has been advancing to windmill blades and various turbines that aim to be improved in electric power efficiency or energy conversion efficiency. About the application of the material to a member large in wall thickness and a large-sized member in each of which many prepregs are laminated onto each other number of laminated sheets of many thick member of the prepreg, apply study to the large member, investigations have been advanced. When such a large-sized structural member is molded, a difference in thermal hysteresis cannot be avoided between its moieties. About such a fiber-reinforced composite material, therefore, even when a fluctuation is generated to some degree in temperature-time profile at the time of molding the material, the material has been required to exhibit morphology and properties equivalent to each other.
Against such problems, suggested is a technique of arranging, in regions between fiber layers (i.e., in interlaminar regions), a particulate material in which, for example, a high-toughness polyamide is used, so as to heighten the resultant workpiece in Mode II interlaminar fracture toughness, and restrain the outer surface of the member from being damaged by falling-weight impact (see Patent Document 1). In this technique, a high-toughness component is located in the form of particles onto the outer surface of a prepreg; thus, the interlaminar fracture toughness can be made high without damaging handleabilities of the prepreg, such as the tackiness or the drape thereof. However, even when this technique is used, the interlaminar particles are deformed in accordance with conditions for molding the fiber-reinforced composite material, so that the interlaminar form is fluctuated. As a result, there remains a problem that the resultant cannot exhibit a stable interlaminar fracture toughness nor impact resistance.
A material is disclosed which exhibits not only a high impact resistance but also a high interlaminar fracture toughness by using a matrix resin containing high-melting-point thermoplastic particles and low-melting-point thermoplastic particles (see Patent Document 2). Even by use of this technique, however, in accordance with conditions for molding the fiber-reinforced composite material, the molded material undergoes the melting or deformation of its interlamilar particles so that the resultant product is varied in interlamilar form. Thus, the product cannot exhibit a stable interlaminar fracture toughness nor impact resistance. Furthermore, a material is disclosed which is improvable in impact resistance and interlaminar fracture toughness while keepable in heat resistance by combining two particle species different from each other in glass transition temperature (Tg) with each other; and an example is disclosed in which complete-sphere-form polyamide particle species different from each other in Tg and particle diameter are combined with each other (see Patent Document 3). Even by use of this technique, however, in accordance with conditions for molding the fiber-reinforced composite material, the molded material undergoes the melting or deformation of its interlamilar particles so that the resultant product is varied in interlamilar form. Thus, the product cannot exhibit a stable interlaminar fracture toughness nor impact resistance.